You do. You have to use Steam for every Valve game on PC. It might seem like a turn off, but Steam is great. You can always redownload your games if you lose the disc. And it's free if you were worried about that.
Wow, if you don't buy Audiosurf at that price... Come on!
Buy it, it's excellent and the developer deserves the support!
I think I'm going to have to cave on Beyond for that price.
edit: I caved, I've been wanting to play BG&E anyway